1. Field of the Invention
The present invention relates to a projection exposure apparatus and method and, more particularly, to a scan type projection exposure apparatus and method used to manufacture semiconductor integrated circuits and liquid crystal devices.
2. Related Background Art
Many conventional apparatuses of this type have correction functions for imaging characteristics because the apparatuses need to maintain high imaging characteristics. Factors which cause the imaging characteristics to vary are changes in external environment such as atmospheric pressure and temperature, and slight absorption of exposure light by a projection optical system. With regard to changes in environment, the atmospheric pressure and the like are monitored by sensors, and correction is performed in accordance with the detection values, as disclosed in, e.g., U.S. Pat. No. 4,687,322. With regard to absorption of exposure light, light energy incident on a projection optical system is measured, and a change in imaging characteristic owing to absorption of exposure light is calculated on the basis of the measurement value, thereby performing correction, as disclosed in, e.g., U.S. Pat. No. 4,666,273. In this known method, light energy incident on the projection optical system through a mask is detected by, e.g., a photoelectric sensor arranged on a substrate stage. In addition to light energy for projection exposure, which is incident from the mask side, light energy is incident on the projection optical system after it is reflected by a photosensitive substrate. This light energy also changes the imaging characteristics of the projection optical system depending on the intensity. With regard to such light energy, for example, as disclosed in U.S. Pat. No. 4,780,747, light reflected by a photosensitive substrate is measured by a photoelectric sensor arranged in an illumination optical system. The sensor receives the light through a projection optical system and a mask, and a total change in imaging characteristic is calculated in consideration of a change in imaging characteristic owing to this reflected light energy. In this method, light reflected by an optical member, a mask pattern, and the like is incident on the photoelectric sensor in the illumination optical system together with light reflected by the substrate. For this reason, a plurality of reference reflecting surfaces having different known reflectances are set on a substrate stage, and the ratio of the respective outputs from the photoelectric sensor, which correspond to the reference reflecting surfaces, is obtained in advance. The reflectance (more accurately, reflection intensity) of the photosensitive substrate is obtained on the basis of this ratio. As described above, since light reflected by a mask pattern is superposed on light reflected by a photosensitive substrate, sensor outputs corresponding to a plurality of reference reflecting surfaces must be obtained every time a mask is replaced. Alternatively, sensor outputs must be measured and registered in advance.
Conventionally, the amount of change in imagining characteristic owing to absorption of exposure light is obtained to perform correction by the above-described methods.
The above conventional scheme has been developed on the basis of a scheme of projecting/exposing the entire mask pattern on a photosensitive substrate (called a batch exposure scheme or a full field scheme). Recently, however, a so-called scan exposure scheme has been developed, in which exposure is performed by illuminating a portion of a pattern area on a mask with a slit-like beam while moving the mask and a photosensitive substrate relative to each other. In this scheme, since the illumination area on a mask is smaller than that in the batch exposure scheme, the amount of image distortion or illuminance irregularity is small. Furthermore, no limitations are imposed on the field size of a projection optical system in the scan direction, and hence large-area exposure can be performed.
In a scan type exposure apparatus, however, energy incident on the projection optical system changes while a mask is scanned with respect to a slit-like illumination beam. For example, such a change occurs because the area of a light-shielding portion (a chromium layer of a pattern) formed on a mask changes in accordance with the position of a slit illumination area on the mask, and hence the amount of energy incident on the projection optical system during a scan exposure operation changes.
In addition, the amount of light reflected by a mask pattern changes in accordance with the position of a mask. Therefore, the detection precision with respect to the amount of energy which is reflected by a photosensitive substrate and incident on the projection optical system inevitably deteriorates in the conventional scheme.
For the above-described reasons, in the conventional schemes correction based on an accurate amount of change in imaging characteristic owing to absorption of exposure light cannot be performed
It is an object of the present invention to provide a projection exposure apparatus of a scan exposure scheme, which can properly correct the imaging characteristics.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a projection exposure apparatus having an illumination optical system for illuminating a mask, on which a predetermined pattern is formed, with light from a light source, a projection optical system for forming an image of the pattern of the mask on a photosensitive substrate, a mask stage for holding the mask and moving the mask within a plane perpendicular to an optical axis of the projection optical system, a substrate stage for moving the photosensitive substrate within a plane conjugate to the plane with respect to the projection optical system, and imaging characteristic correction means for correcting an imaging characteristic of the projection optical system, the apparatus synchronously moving the mask and the photosensitive substrate along an optical axis of the projection optical system so as to expose an entire pattern surface of the mask, and the apparatus including:
incident light intensity input means for inputting an intensity of the illumination light, which is incident on the projection optical system through the mask, in accordance with a position of the mask,
imaging characteristic calculation means for calculating a variation in imaging characteristic of the projection optical system on the basis of information from the incident light intensity input means; and
control means for controlling the imaging characteristic correction means on the basis of a result obtained by the imaging characteristic calculation means.
According to a second aspect of the present invention, there is provided a projection exposure apparatus having an illumination optical system for illuminating a mask, on which a predetermined pattern is formed, with light from a light source, a projection optical system for forming an image of the pattern of the mask on a photosensitive substrate, a mask stage for holding the mask and moving the mask within a plane perpendicular to an optical axis of the projection optical system, a substrate stage for moving the photosensitive substrate within a plane conjugate to the plane with respect to the projection optical system, and imaging characteristic correction means for correcting an imaging characteristic of the projection optical system, the apparatus synchronously moving the mask and the photosensitive substrate along an optical axis of the projection optical system so as to expose an entire pattern surface of the mask, and the apparatus including:
incident light intensity input means for inputting an intensity of the illumination light, which is incident on the projection optical system through the mask, in accordance with a position of the mask;
reflected light intensity input means for inputting an intensity of the illumination light, which is reflected by the photosensitive substrate and incident on the projection optical system again, in accordance with a position of the mask;
imaging characteristic calculation means for calculating a variation in imaging characteristic of the projection optical system on the basis of information from the incident light intensity input means and information from the reflected light intensity input means; and
control means for controlling the imaging characteristic correction means on the basis of a result obtained by the imaging characteristic calculation means.
According to the present invention, even if energy incident on the projection optical system changes when a mask is scanned during an exposure operation, no problem is posed because illumination light intensity data corresponding to the position of the mask can be used for calculation of a variation in imaging characteristic caused by absorption of exposure light. In addition, according to the present invention, a variation in imaging characteristic owing to absorption of exposure light can be accurately obtained because energy incident on the projection optical system is calculated in consideration of information about light reflected by the photosensitive substrate.
As described above, according to the present invention, since a variation in imaging characteristic can be accurately calculated on the basis of the amount of energy incident on the projection optical system which changes in accordance with the position of a mask, the imaging characteristic can be corrected without any error even in a scan type exposure apparatus.